The invention relates to a process for preparing wood-fibre materials in a particularly economical manner and where the end product is given a previously not achieved range of properties.
The present demand for flat moulded articles, boards, webs and the like is satisfied with various suitable materials, and combinations thereof, which are prepared and processed by many and varied techniques.
Essential sources of raw material for such materials are homegrown, renewable types of wood which are treated and processed to meet the demands placed on the finished product and with an eye on their particular, specific properties. Demands which these materials have to meet are of course primarily the result of the need for economical manufacture and, in some cases, for a long life given sensible use of the parts.
The catalogue of criteria for determining fitness for use is complex, and includes, inter alia, requirements such as universal shapeability, fire resistance, thermal and water stability at a suitable level of water absorption, freeze-thaw cycle stability, resistance to root penetration, acoustic and thermal insulating properties, low density, high flexural tensile strength and rigidity, high compressive strength and strength transverse to the grain, laminatability and/or paintability, gas permeability, uniform surface appearance, pleasant feel and sound, physiological acceptability, force-induction capability and screw-holding power, reliable reproducibility, long-term availability of the raw materials, and so on and so on.
It has been found that if the known methods of preparing flat moulded articles based on, for example, wood materials satisfy one requirement of their properties, they are unable to satisfy an examination in respect of the other requirements. For this reason those skilled in the art have for a long time had to accept compromise solutions which were the result of apparently diametrically opposed requirements, or the overall level of the properties had to be raised, by an economically not always justified high degree of refinement of the products, in order, then, to satisfy a few demands fully.
The existing wet-moulding processes combine very low binder contents with fibre structures which are on the one hand well-felted, hence having substantial strength while there is considerable freedom in designing the shape, but which, on the other hand, require energy-intensive squeezing, drying and compression-moulding steps, which are expensive. Moreover, the finished products are always marked on one face by the pattern of the drying sieve.
The existing dry methods, at comparable binder contents and without incorporating relatively expensive auxiliary materials and carriers, give inadequate fibre structures, so that obtaining good moulding results always necessitates resin-rich batches. Also required are modifying additives, preliminary and/or intermediate and/or after-treatments, such as cutting the mat to size, moisture or steam opening treatments, impregnating, dwell times for cooling and/or relaxing, and the like.
It follows from this need that it is an object of the present invention to prepare as economically as possible flat fibrous moulded articles which satisfy a very broad spectrum of different technical requirements.